Here are the ways the Eclipse documentation states you can navigate the Content Assist list:
You can use the mouse or the keyboard (Up Arrow, Down A
This is a great feature, and you can implement it like so: bind movement keys in Eclipse's key binder and then use them in the content assist menu!
Preferences/General/Keys
Line Down: Bind to Ctrl+K Line up: Bind to Ctrl+I
(both when 'Editing Java Source')
and then try moving with Ctrl-K/I when the content assist pops up!
In Kepler, as jed mentions in the comments, select "when editing text" instead and it should work.